The present invention relates to a multifocal ophthalmic lens, having an aspherical surface which has a mean sphere and a cylinder at every point thereon.
Such lenses are well known; among multifocal lenses one can distinguish lenses known as progressive lenses adapted to vision at all distances, and lenses that are more specifically dedicated to near vision and intermediate vision.
Progressive multifocal ophthalmic lenses comprise a far vision region, a near vision region, an intermediate vision region, and a main meridian of progression passing through the three regions. French patent application 2,699,294, which is incorporated herein by reference, describes, in its preamble, the various elements of a progressive multifocal ophthalmic lens (main meridian of progression, far vision region, near vision region, etc.), as well as the work carried out by the applicant to improve wearer comfort of such lenses.
Applicant has also proposed, in order to better satisfy the visual needs of long sighted people and to improve comfort of progressive multifocal lenses, to adapt the shape of the main meridian of progression, as a function of the power addition value A (French patent application FR-A-2,683,642).
For such lenses, the power addition value A is defined as the variation in mean sphere between a reference point in the far vision region and a reference point in the near vision region.
Such progressive lenses are generally prescribed as a function of the wearer""s ametropia and the power needed for near vision.
Lenses also exist which are more specifically dedicated to near vision; such lenses do not have a a far vision region with a defined reference point like conventional progressive lenses have. Such lenses are prescribed depending on the near vision power the wearer needs, independently of the far vision power. Such a lens is described in an article in the xe2x80x9cOpticien Lunetierxe2x80x9d dated April 1988, and is sold commercially by the applicant under the Essilor Delta trademark; this lens is also as simple to use and easy to wear as a progressive lens, and it is attractive to long-sighted people not fitted with progressive lenses. This lens is also described in French patent application FR-A-2,588,973. It has a central portion which is equivalent to the single-focus lens that would normally have been employed for correcting longsightedness, in order to ensure satisfactory near vision. It additionally has a slight decrease in power in the upper portion, ensuring that the wearer also has sharp vision beyond the usual near vision field. Finally, the lens has a point at a power value equal to the nominal power for near vision, a higher power region at the lower portion of the lens, and a lower powered region in the top portion of the lens.
Existing multifocal lenses, whether they be progressive or dedicated to near vision can still be further improved as regards their foveal vision performance, in order, to improve wearer comfort. Wearers of multifocal lenses do in fact sometimes feel uncomfortable with dynamic vision. Such lenses can also be improved by preserving a near vision region that is sufficiently high to ensure optimum wearer comfort; finally, it is important that wide visual fields be provided in near, intermediate and far vision.
The present invention provide a multifocal lens which overcome the disadvantages of prior art lenses and which provides wearers with improved peripheral vision while still ensuring foveal vision is good, thereby ensuring ease of adaptation of wearers to their lenses. The invention nevertheless ensures rapid progression of mean sphere, ensuring the presence of a large near vision region. It also provides balanced distribution of isosphere and isocylinder lines.
The invention provides a multifocal ophthalmic lens comprising an aspherical surface having at every point thereon a mean sphere and a cylinder, and comprising a far vision region VL, a near vision region VP, an intermediate vision region VI, a main meridian of progression MMxe2x80x2 passing through said three regions,
in which a principal length of progression, defined as a ratio between power addition and maximum slope of mean sphere along said meridian is less than 16 mm;
in which sphere varies in a monotonous fashion as a function of angle on a 20 mm radius circle centered on a geometric center of the lens at both sides of said meridian,
and in which the far vision region delimited in an upper portion of said lens by lines formed of points for which cylinder is equal to half power addition includes an angular sector having its origin at the geometric center of the lens with an included angle greater than 150xc2x0.
The principle length of progression can preferably fall within ranges that have about 15 mm, about 14 mm, or about 13 mm as an upper limit. The lower limit for such ranges can be, for example, about 12 mm, about 11 mm, or about 10 mm. Most preferably, the principle length of progression is about 12 mm, i.e., in the range of about 12 to 13 mm.
Advantageously, the main meridian of progression is made up by mid-points of horizontal segments joining respective lines formed by points where cylinder is 0.50 diopter.
In one embodiment, the near vision region, delimited in an upper portion of said lens by lines formed by points where cylinder is equal to half power addition has a width that is greater than 12 mm at a point of reference for near vision.
In another embodiment, said included angle has a value comprised between 160xc2x0 and 170xc2x0, preferably of the order of 165xc2x0.
Preferably, the modulus of the derivative dS/d"THgr" of mean sphere with respect to angle on said circle is comprised between 0.005 and 0.015 when said angle "THgr" is comprised in the ranges [30xc2x0; 100xc2x0] and [270xc2x0; 325xc2x0].
Advantageously, the modulus of the derivative dS/d"THgr" of mean sphere with respect to angle on said circle is comprised between 0.01 and 0.04 when said angle "THgr" is comprised in the ranges [125xc2x0; 187xc2x0] and [187xc2x0; 250xc2x0].
In one embodiment, the lens is a multifocal lens dedicated to near vision and intermediate vision, said lens having a power addition defined as a difference between maximum and minimum values of mean sphere on said meridian of progression, inside a 20 mm radius circle centered on the geometric center of said lens.
In another embodiment, the lens is a progressive multifocal lens having a reference point for a near vision region, a reference point for a far vision region, and a power addition defined as a difference between the values of mean sphere at these two points.
Further features and advantages of the present invention will become more clear from the description which follows of one embodiment of the invention provided by way of non-limiting example with reference to the attached drawings.